| Summary: | The objective of this study was to investigate whether a tipping point exists in the calcification responses of coral reef calcifiers to CO 2 . We compared the effects of six partial pressures of CO 2 (P CO2 ) from 28 Pa to 210 Pa on the net calcification of four corals ( Acropora pulchra, Porites rus, Pocillopora damicornis, and Pavona cactus ), and four calcified algae ( Hydrolithon onkodes, Lithophyllum flavescens, Halimeda macroloba, and Halimeda minima ). After 2 weeks of acclimation in a common environment, organisms were incubated in 12 aquaria for 2 weeks at the targeted P CO2 levels and net calcification was quantified. All eight species calcified at the highest P CO2 in which the calcium carbonate aragonite saturation state was ???1. Calcification decreased linearly as a function of increasing partial P CO2 in three corals and three algae. Overall, the decrease in net calcification as a function of decreasing pH was ???10% when ambient P CO2 (39 Pa) was doubled. The calcification responses of P. damicornis and H. macroloba were unaffected by increasing P CO2 . These results are inconsistent with the notion that coral reefs will be affected by rising P CO2 in a response characterized by a tipping point. Instead, our findings combined among taxa suggest a gradual decline in calcification will occur, but this general response includes specific cases of complete resistance to rising P CO2 . Together our results suggest that the overall response of coral reef communities to ocean acidification will be monotonic and inversely proportional to P CO2 , with reef???wide responses dependent on the species composition of calcifying taxa.
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